Recently, a magentic recording medium has been required to be more sensitive. If the magnetic recording medium is made more sensitive, a high quality image as well as a good sound quality and high density recording thereof can be realized. Those improvements can be attained by improving the deck for magnetic recording and replaying and the recording system of a magnetic recording medium, and also by improving a magnetic recording medium itself.
A magnetic recording medium can be highly sensitized by increasing the signal and decreasing the noise in the signal/niose ratio thereof. The signal can be increased by increasing a residual magnetic flux density and a coercive force of the magnetic recording medium, which can be realized by making ferromagnetic particles finer or adjusting the shape of anisotropy and arrangement of a magnetic single domain of each ferromagnetic fine particle.
The noise can be decreased by various factors, for example, by smoothing the magnetic layer and controlling the charging properties of the magnetic recording medium. It has been disclosed that a backing layer can be provided in order to control the charging properties of the magnetic recording medium and maintain the running durability thereof, for example, in Japanese Patent Application (OPI) Nos. 74909/1974, 16105/1972, 75102/1974, 75103/1974, 11305/1974, 48109/1973, U.S. Pat. Nos. 3,617,378, 3,734,772, 3,196,039, 4,135,031, British Pat. No. 1,198,009, Japanese Patent Publication Nos. 2612/1965, 8321/1974, 10244/1974, 2415/1983, Japanese Patent Application (OPI) Nos. 3132/1983, 130234/1982, Japanese Patent Publication No. 3927/1975, Japanese Patent Application (OPI) Nos. 150132/1982, 123532/1982, 161135/1983, 96505/1977, and Japanese Patent Publication No. 23647/1983.
However, a magnetic recording medium having high sensitivity and sufficient running durability cannot be obtained satisfactorily by the above technology. This is because (1) although the surface properties of the magnetic recording medium can be improved by smoothing the surface thereof, and a spacing loss between the magnetic recording and replaying head and the magnetic recording medium can be reduced, the running durability thereof deteriorates and the friction coefficient increases, whereby the magnetic recording layer and the backing layer are worn out; (2) if the surface of the backing layer is made rough in order to improve the running durability of the backing layer, the rough surface of the backing layer is printed through on the magnetic layer, thereby the signal/noise ratio of the magnetic layer decreases; and (3) at an initial stage of running when the magnetic layer and the backing layer are not yet worn out, drop outs increases, thereby decreasing the signal/noise ratio.
The present inventors made thorough investigations and, as the result, have found that the drop outs occurring at the initial stage and after repeated use is caused by dust and contaminants, which are adhered on the surface of the magnetic recording medium and scrapes of the magnetic recording medium. In many of these cases, the adhesion of dust and contaminants have effect the insulating or non-insulating properties of the tape path of the magnetic tapes on the deck for magnetic recording and replaying, the running speed and the running tension of the magnetic tape. Particularly in the magnetic recording medium, the adhesion of the dust and contaminants effect the charging property, the surface electric resistance and the friction coefficient of the magnetic recording medium. By properly adjusting the hardness and toughness of the materials of which the magnetic recording medium is composed and the surface roughness of the magnetic recording medium, a magnetic recording medium having a high signal/noise ratio can be realized. Further, the surroundings are important where the magnetic recording medium is run, because they have a close and complicated relationship to the charging properties of the dust and contaminants and the temperature and humidity which effect on the charging properties thereof.
Factors affecting the increase in drop outs due to scraping of the magnetic recording medium are that the magnetic layer or the backing layer drops off as powder caused by the friction between the magnetic recording medium and the tape running path in its container or recording/replaying device and that bumps of the dust and contaminants which adhere on the tape running path scrape off the magnetic layer and the backing layer of the magnetic recording medium. In order to prevent the scraping off of the backing layer by the dust and contaminants, it is necessary that the backing layer has a thickness larger than the diameter of the bumps and that a sufficiently tough binder is well mixed and kneaded with the particles in order to prevent the peeling off of the backing layer and the magnetic layer. In order to prevent the scraping off of the backing layer by the bumps of the dust and contaminants, it is necessary that the backing layer has a thickness of from 1.0 to 3.0 .mu.m. When the backing layer has a thickness of 1.0 .mu.m or less, generally the dust and contaminants which are present in the air readily scrape off the backing layer in a running direction of the magnetic recordng medium, whereby scratches are made continuously in that direction and surface unevenness is generated.